Topic 8 Iterators
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1 Topic 8 Iterators "First things first, but not necessarily in that order " -Dr. Who
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Topic 8 Iterators. "First things first, but not necessarily in that order " -Dr. Who. Iterators. ArrayList is part of the Java Collections Framework Collection is an interface that specifies the basic operations every collection (data structure) should have - PowerPoint PPT Presentation
Transcript of Topic 8 Iterators
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Topic 8Iterators
"First things first, but not necessarily in that order "
-Dr. Who
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Iterators ArrayList is part of the Java Collections
Framework Collection is an interface that specifies the
basic operations every collection (data structure) should have
Some Collections don’t have a definite order– Sets, Maps, Graphs
How to access all the items in a Collection with no specified order?
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Iterator Interface An iterator object is a “one shot” object
– it is designed to go through all the elements of a Collection once
– if you want to go through the elements of a Collection again you have to get another iterator object
Iterators are obtained by calling a method from the Collection
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Iterator Iterface Methods The Iterator interface specifies 3 methods:
boolean hasNext()//returns true if this iteration has more elements
E next()//returns the next element in this iteration//pre: hastNext()
void remove()/*Removes from the underlying collection the last element
returned by the iterator. pre: This method can be called only once per call to next.
After calling, must call next again before calling remove again.
*/
Clicker Question 1 Which of the following produces a syntax error?ArrayList<String> list
= new ArrayList<String>(); Iterator<String> it1 = new Iterator(); // IIterator<String> it2 = new Iterator(list); // II Iterator<String> it3 = list.iterator(); // III
A. IB. IIC. IIID. I and IIE. II and III
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Iterator Imagine a fence made up of fence posts and
rail sections
fenceposts
rails
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Fence Analogy The iterator lives on the fence posts The data in the collection are the rails Iterator created at the far left post As long as a rail exists to the right of the
Iterator, hasNext() is trueiterator object
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Fence AnalogyArrayList<String> names =
new ArrayList<String>();names.add(“Jan”);names.add(“Levi”);names.add(“Tom”);names.add(“Jose”);Iterator<String> it = names.iterator();int i = 0;
“Jan” “Levi” “Tom” “Jose”
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Fence Analogywhile( it.hasNext() ) {
i++;
System.out.println( it.next() );
}
// when i == 1, prints out Jan
“Jan” “Levi” “Tom” “Jose”
first call to next moves iterator tonext post and returns “Jan”
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Fence Analogywhile( it.hasNext() ) {
i++;
System.out.println( it.next() );
}
// when i == 2, prints out Levi
“Jan” “Levi” “Tom” “Jose”
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Fence Analogywhile( it.hasNext() ) {
i++;
System.out.println( it.next() );
}
// when i == 3, prints out Tom
“Jan” “Levi” “Tom” “Jose”
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Fence Analogywhile( it.hasNext() ) {
i++;
System.out.println( it.next() );
}
// when i == 4, prints out Jose
“Jan” “Levi” “Tom” “Jose”
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Fence Analogywhile( it.hasNext() ) {
i++;
System.out.println( it.next() );
}
// call to hasNext returns false
// while loop stops
“Jan” “Levi” “Tom” “Jose”
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Typical Iterator Patternpublic void printAll(Collection<String> list) {
Iterator<String> it = list.iterator();while( it.hasNext() ) {
Object temp = it.next();
System.out.println( temp );
}
}
Clicker Question 2 What is output by the following code?ArrayList<Integer> list;
list = new ArrayList<Integer>();
list.add(3);
list.add(3);
list.add(5);
Iterator<Integer> it = list.iterator();
System.out.println(it.next());
System.out.println(it.next());
A.3 B. 5 C. 3 3 5
D. 3 3 E. 3 5CS314
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remove method AnIterator can be used to remove things from
the Collection Can only be called once per call to next()public void removeWordsOfLength(int len)
{ Iterator<String> it = myList.iterator
while( it.hasNext() ) { String temp = it.next();
if(temp.length() == len) it.remove();
}
}// original list = [“dog”, “cat”, “hat”, “sat”]
// resulting list after removeWordsOfLength(3) ?
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Clicker Question 3public void printTarget(ArrayList<String> names,
int len) {
Iterator<String> it = names.iterator();
while( it.hasNext() )
if( it.next().length() == len )
System.out.println( it.next() );
}
Given names = [“Jan”, “Ivan”, “Tom”, “George”] and len = 3 what is output by the printTarget method?
A. Jan Ivan Tom George
B. Jan Tom
C. Ivan George
D.No output due to syntax error
E.No output due to runtime error
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The Iterable Interface A related interface is Iterable One method in the interface:
public Iterator<T> iterator() Why? Anything that implements the Iterable
interface can be used in the for each loop.ArrayList<Integer> list;//code to create and fill listint total = 0;for( int x : list ) total += x;
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Iterable If you simply want to go through all the
elements of a Collection (or Iterable thing) use the for each loop– hides creation of the Iterator
public void printAllOfLength(ArrayList<String> names, int len){
//pre: names != null, names only contains Strings //post: print out all elements of names equal in // length to len
for(String s : names){if( s.length() == len )
System.out.println( s ); }
}
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Implementing an Iterator Implement an Iterator for our GenericList
class– Nested Classes– Inner Classes– Example of encapsulation– checking precondition on remove– does our GenricList need an Iterator?
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Comodification If a Collection (ArrayList) is changed
while an iteration via an iterator is in progress an Exception will be thrown the next time the next() or remove() methods are called via the iterator
ArrayList<String> names =
new ArrayList<String>();
names.add(“Jan”);
Iterator<String> it = names.iterator();
names.add(“Andy”);
it.next(); // exception will occur here
